1. Field of the Invention
The present invention relates to a dynamic pressure-type fluid bearing unit used in a rotator device such as a magnetic disc drive, and a spindle motor using the same. More particularly, the present invention relates to a fluid bearing unit having a feature in means for reducing bearing torque and motor current consumption.
2. Description of the Related Art
A fluid bearing unit comprises a shaft and a bearing for receiving the shaft. As the fluid bearing unit rotates, a lubricant in a gap between the shaft and the bearing is collected by dynamic pressure generating grooves formed in one of the shaft and the bearing, so that pressure is generated in the gap and the shaft is supported by the bearing in a non-contact manner.
A spindle motor provided with the fluid bearing unit is excellent in rotational accuracy which is indispensable in improving the recording density of a medium, impact resistance and noise reduction. Therefore, the spindle motor is mainly used for a driving device, typically, a magnetic disc drive of information equipment and AV equipment.
Recently, the spindle motor has a strong demand of reduction in motor current consumption, particularly, reduction in torque of the fluid bearing unit constituting most of the current consumption in view of miniaturization and energy-saving of the equipment. The torque of the fluid bearing unit is proportional to the viscosity of a lubricant to be filled; therefore, the lubricant having lower viscosity becomes necessary. Consequently, an ester derived from neopentyl glycol and monovalent fatty acid having 6 to 12 carbon atoms and/or derivatives thereof is used as a base oil which constitutes a main component of the lubricant (see, for example, JP2001-316687 A). Further, a fluid bearing unit using monoester and the like is proposed (see, for example, JP2000-63860 A).
However, the conventional fluid bearing unit using such lubricant has a problem in that it can not sufficiently achieve the torque reduction as desired since the size of the bearing involved in miniaturization is limited.
Further, in the case of using a monoester-based lubricant, the torque reduction in the bearing can be achieved. However, since the monoester-based lubricant normally has a high pour point, there is a possibility in that the fluidity may be lost at a temperature of 0° C., moreover, at a low temperature region of not more than −20° C. and the lubricant may be solidified. In the case of using the lubricant in a fluid bearing unit for portable equipment or in-vehicle equipment, the necessary torque of the bearing becomes considerably high under the necessary environment of not more than −40° C., so that there is a problem in that the bearing unit cannot be rotationally activated and the range of the operating temperature is limited.